Variable-speed transmissions with two countershafts and a central main shaft are known from the prior art. Main shaft gearwheels arranged to rotate on the main shaft are driven by the gearwheels on the countershafts. Only when operating in the associated gear are the main shaft gearwheels connected in a rotationally fixed manner to the main shaft. In other operating phases the main shaft gearwheels rotate loosely on the main shaft. Thus, the main shaft gearwheels are also called loose gearwheels.
For the purpose of load equalization between the force flows through the two countershafts, in such transmissions the main shaft is mounted in as floating manner, i.e. with some radial play. The loose gearwheels are always axially fixed on the floating main shaft, but not in the radial direction. In the radial direction the loose gearwheels are guided and mounted in the teeth of the countershaft gearwheels that mesh with the loose wheels. This achieves optimum load equalization.
The loose wheels of gears not engaged at the time are in the load-free condition and float within the tooth play, i.e. they have some play in the radial and circumferential direction relative to the main shaft and relative to the gearwheels of the countershafts.
However, due to the play of the loose wheels in the load-free condition an unpleasant rattling noise is produced during operation because the gearwheels undergo accelerations in the circumferential and radial directions, because of manufacturing imprecisions and by rotation fluctuations from the drive input and/or the drive output. Owing to these accelerations the loose gearwheels move about within the movement clearance range described and impact the adjacent components. For example, the tooth flanks of teeth engaged with one another strike one another within the range of their tooth flank clearance, and this is the main cause of the rattling from the gearwheels co-rotating while not under load.
Solutions have already been proposed for reducing rattle noise in variable-speed transmissions with two countershafts. For example, from DE 102004057126 A1 a device for reducing the axial movement of the loose gearwheels in a transmission with two countershafts is known, such that a pressing device arranged on the main shaft presses one of the loose wheels in the axial direction against a buffer disk. This axial bracing of the loose wheels is intended to prevent undesired rattling noise. With a pressing device as described in DE 102004057126 A1 sliding friction takes place during operation between the pressing device and the loose wheel. This sliding friction increases the friction losses of the transmission and results in wear.
The purpose of the present invention is to provide a variable-speed transmission with two countershafts, which operates at a low noise level and with low friction losses.